Light
Color Rendering Index
CRI
A 0–100 score for how faithfully a light source renders colors against a reference. It averages eight pastel samples, which makes it a weak predictor of saturated-color performance — the gap TM-30 was created to close.
In more detail
CRI scores how faithfully a light source renders colors compared with a reference illuminant at the same CCT. The general index, Ra, averages results across eight moderately saturated pastel test samples and reports a number up to 100.
The averaging is the weakness. Because the eight samples are pastel, a source can score well while rendering saturated colors poorly — deep reds especially, which is why R9, an extended sample not included in Ra, is often quoted separately. A lamp at Ra 90 with a low R9 makes wood, skin and red fabric look flat.
IES TM-30 was developed to address this, using 99 samples and reporting fidelity (Rf) and gamut (Rg) separately along with per-hue detail. It is the more complete evaluation and is increasingly specified. CRI remains on nearly every product datasheet, which makes it the number most often used and the one least able to distinguish between good sources at the top of its range.
Why it matters
CRI is where lighting decisions quietly become interior finish decisions. A house full of Ra 82 lamps has slightly grey skin tones, muted wood and washed-out textiles, and the cause is rarely identified — the client concludes the paint color was wrong. Specifying high fidelity, including deep red performance, costs little at fixture selection and cannot be corrected later without replacing lamps.
The common misunderstanding
That Ra 90 or above is sufficient information. Ra stops discriminating well above roughly 90, and two sources with the same Ra can look clearly different on saturated colors. Where they are published, TM-30's Rf and Rg pair — or at minimum R9 — says what Ra alone cannot.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.